JPH07241520A - Method for making designed coating film - Google Patents

Abstract

PURPOSE:To provide a method for making a designed coating film having high saturation, no turbidity in shade, a good pearl tone reproducing the hue of a colored substrate. CONSTITUTION:A double-layered coating film is formed by a sequential process of the formation of (1) a colored-substrate coating film of brightness 3 or more, hue 2 or more in a Munsell color system, (2) a bright-layered coating film containing 3-10 pts.wt. of bright pigment comprising flake type barium sulfate pigment coated with titanium dioxide and mica pigment coated with titanium dioxide in a weight ratio of 95/5-50-50 (P1/P2) per 100 pts.wt. of solid resin, and (3) a clear coating film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming a design coating film useful for industrial coating of automobiles and parts thereof, bicycles, home electric appliances, etc. The present invention relates to a method for forming a multi-layer coating film which is excellent in design and gives a pearlescent luster that reproduces the color of a colored substrate.

[0002]

2. Description of the Related Art Conventionally, silver mica having a thin film of titanium dioxide coated on the surface of scaly mica and interference mica titanium have been used as a paint component which gives a pearlescent luster to the coating film. The coating film is formed by a three-coat type multilayer coating film forming method in which a paint containing the mica pigment is applied onto a colored undercoating film and then a clear coat is formed.

As a coating technique for a coating system containing this type of silver and interference mica titanium, for example, a particle size of 5 to 5 is used.
A lacquer coating method using a pearlescent pigment in which a lacquer containing a titanium dioxide-coated mica pigment of 25 μm is further coated, and a clear lacquer is further coated (Japanese Patent Laid-Open No. 58-166964), Munsell color charts N-4 to N-8. A coating method in which a transparent pearl coating film containing mica coated with 6 to 13% (resin solid content ratio) of a metal oxide mainly composed of titanium dioxide is coated on a color base, and a top clear coat is further coated thereon (JP-A-2006-242242). 61-37423 gazette), when overcoating a low-hiding paint containing a titanized mica pigment on a colored undercoating film, the pigment interference color of the titanized mica pigment should be the same type as the hue of the colored undercoating film. A coating finishing method to be used (JP-A-59-160571) has been proposed.

As a base material for these titanium dioxide-coated mica pigments, natural muscovite flakes having a smooth surface are selectively used in order to obtain excellent glitter. However, natural muscovite contains a small amount of iron, mud, etc. as impurities, and when these components are baked to convert titanium hydroxide into TiO ₂ , it is oxidized and becomes pale yellow or slightly yellowish brown. Cause a phenomenon to occur. When such a phenomenon occurs, the pure whiteness peculiar to muscovite is obstructed, and the shade portion gives a sensation of cloudy bottom.

In order to solve such a problem, a pearly luster composition comprising a barium sulfate pigment coated with titanium dioxide, which is a flaky barium sulfate single crystal having high transparency as a coating substrate, and a method for producing the same are provided by the present applicant. It is being developed by one person (Japanese Patent Publication No. 47-24930).

[0006]

The above-mentioned titanium dioxide-coated flaky sulfate barium pigment is superior in transparency as compared with titanium dioxide-coated mica pigment, and is a pure single crystal, so that it contains impurities causing coloring. It is not. Therefore, although the problem of hue when using the titanium dioxide-coated mica pigment described above is solved, there is a drawback that the luster is poor.

The present inventors have conducted extensive studies to solve the drawbacks inherent in the above titanium dioxide-coated mica pigment and titanium dioxide-coated flaky barium sulfate pigment. As a result, these titanium dioxide-coated mica pigment and dioxide Titanium-coated flaky barium sulfate pigment is blended in a specific amount range to form a paint, which is applied to a colored undercoating surface having a certain lightness and saturation, and a clear coating film is formed on the upper surface. It was confirmed that the individual components complement each other's defects to develop a pearlescent luster feeling that shows the hue of the colored base.

The present invention has been developed on the basis of the above-mentioned findings, and its purpose is to provide an excellent pearlescent luster that reproduces the color tone of a colored base with high saturation and no cloudiness in the shade portion. It is to provide a method for forming a design coating film that gives

[0009]

[Means for Solving the Problems] A method for forming a design coating film according to the present invention for achieving the above object is described in the following (1) to (3)
The constitutional feature is to sequentially perform the coating film forming step. (1) Brightness is 3 or more and saturation is 2 in the Munsell display system
The above-mentioned colored base layer coating film forming step (2) The titanium dioxide-coated flaky sulfate sulfate pigment (P1) and the titanium dioxide-coated mica pigment (P2) having a weight ratio (P1 / P2) in the range of 95/5 to 50/50. (3) A bright layer coating film forming step containing 3 to 10 parts by weight of a bright pigment having a composition of (3) per 100 parts by weight of a resin solid content, and (3) a clear coating film forming step.

The object to be coated on which the design coating film of the present invention is formed includes metals such as iron, aluminum, copper or alloys thereof, inorganic materials such as glass, cement and concrete, polyethylene, polypropylene and ethylene. -Vinyl acetate copolymer, polyamide, polyacryl, polyester, ethylene-polyvinyl alcohol copolymer, resin molded products such as vinyl chloride, vinylidene chloride, polycarbonate, polyurethane, and plastic materials such as various FRP, wood and fiber materials To do. In addition, it is optional to perform appropriate undercoating or precoating treatment on these coated substrates in advance.

The coating can be applied directly to the object to be coated. For example, in the case of coating on automobiles, usually, an undercoat coating such as an electrodeposition coating is applied after the surface chemical conversion treatment, and the coating is cured before the coating is applied. Form. As the coating method, atomization type coating and roll coater type coating are used, but air spray coating by the atomization type coating method, electrostatic coating and the like are particularly preferably used.

First, a colored base coating film, which is generally called a color intermediate coating material, is formed on an object to be coated. As the pigment used in the color intermediate coating material, various organic and inorganic coloring pigments and extender pigments that are commonly used for coating materials can be used.
As the coloring pigment, for example, an organic azo lake pigment,
Inorganic yellow lead such as insoluble azo pigment, condensed azo pigment, phthalocyanine pigment, indigo pigment, perinone pigment, perylene pigment, dioxazine pigment, quinacridone pigment, isoindolinone pigment, metal complex pigment ,
Yellow iron oxide, red iron oxide, carbon black, titanium dioxide and the like are used as extender pigments such as calcium carbonate, barium sulfate, clay and talc. Examples of the resin that serves as a vehicle for forming a coating film include a system in which a basic resin such as an acrylic resin, a polyester resin, or an alkyd resin is mixed with a crosslinking agent such as an amino resin or (block) polyisocyanate. In addition, a two-component polyurethane resin or silicone resin that can be cured by drying at room temperature is also used. The dry film thickness of the colored undercoat film to be formed is in the range of 25 to 50 μm, preferably 30 to 40 μm.

The colored undercoating film formed in this step is controlled so that the hue of the coating film is 3 or more and the chroma is 2 or more in the Munsell display system by controlling the mixing ratio of the color pigment and the vehicle resin. Adjust to. If the lightness is less than 3, white blurring or color unevenness occurs, and if the saturation is less than 2, the feeling of bottom turbidity increases, and in any case, the color tone of the colored base is changed. It becomes impossible to express the pearlescent brightness that is effectively reproduced. A particularly preferable hue of the colored undercoating film has a Munsell display system having a lightness of 6 or more and a saturation of 4
That is all.

On the surface of the colored undercoat film, a paint containing a bright pigment containing a titanium dioxide-coated flaky sulfate pigment and a titanium dioxide-coated mica pigment is applied by dry-on-wet to form a glitter layer coating film. Depending on the process, wet-on-wet may be used.

The titanium dioxide-coated flaky barium sulfate pigment (P1), which constitutes the glitter coating, has a coating composition in which titanium dioxide is coated as a thin layer on the surface of flaky barium sulfate single crystal powder. Can be prepared in this manner. Sulfate concentration is 0.5-0.001mol /
1 of soluble sulfate solution having a barium salt concentration of 0.5 to 0.
Add 001mol / l soluble barium salt solution and react under normal temperature to boiling point under acidic pH range to obtain 5
A flaky single crystal of barium sulfate having a thickness of about 100 μm and a thickness of about 0.05 to 1 μm is produced. For example, p with hydrochloric acid
When H3 was adjusted and 10 l of a 0.01 mol solution of ammonium sulfate heated to 40 ° C. was added to 5.5 l of a 0.02 mol solution of barium chloride heated to 40 ° C. and reacted, the major axis was 5 to 100 μm, Slightly deformed barium sulphate rhombic crystals with flaky aspect properties with minor axis of 5 to 30 μm are formed. The major axis is preferably 10 to 40 μm and the minor axis is 5 to 30 μm.

The flaky barium sulfate thus obtained is washed with water, classified and then suspended in pure water. Then, titanyl sulfate, titanium tetrachloride and other water-soluble titanium compounds or complex compounds are added and hydrolyzed by heat or alkali. The titanium hydroxide adhered to the surface of the flaky barium sulfate by this hydrolysis is washed with water and then subjected to a firing treatment to form a thin layer of titanium dioxide. In this step, a metal oxide such as tin or zirconium may be added to the extent that coloring is not hindered. In this way, a flaky titanium dioxide-coated flaky barium sulfate pigment having a pearly luster and interference is obtained.

On the other hand, as the titanium dioxide-coated mica pigment (P2), a mica fine powder coated with a thin layer of titanium dioxide to give a pearlescent effect is used, and it is used in combination with the above-mentioned titanium dioxide-coated flaky barium sulfate pigment. Be compounded. The compounding ratio of the titanium dioxide-coated flaky barium sulfate pigment (P1) and the titanium dioxide-coated mica pigment (P2) is the weight ratio (P1 /
P2) is set in the range of 95/5 to 50/50. When the compounding ratio of the titanium dioxide-coated mica pigment is less than the above range, the brilliance is reduced, and when it exceeds the above range, the feeling of bottom turbidity is enhanced and the hue of the colored base coating film becomes turbid. A more preferable blending weight ratio (P1 / P2) is 80/20 to 50/50.
Is the range.

A bright pigment having a composition in which a flaky barium sulfate pigment coated with titanium dioxide and a mica pigment coated with titanium dioxide are mixed with a vehicle component to prepare a bright coating material.
As a vehicle resin, a base resin such as an acrylic resin, a polyester resin, an alkyd resin, a silicon resin, or a fluororesin is mixed with a cross-linking agent such as an amino resin or (block) polyisocyanate, and these resins are used. Not only one kind but also two or more kinds may be used in combination. In addition, a two-component polyurethane resin or silicone resin that can be cured by drying at room temperature is also used.

The amount of the bright pigment added to the vehicle resin is 3 to 10 parts by weight, preferably 4 to 8 parts by weight, based on 100 parts by weight of the resin solid content. If the amount added is less than 3 parts by weight, the interference color and the brilliance will be deteriorated, and if it exceeds 10 parts by weight, the brilliance will be too strong and the image will be blurred or the reproduction of the colored background color will be reduced. The thickness of the bright layer coating film formed is preferably in the range of 10 to 30 μm as a dry film thickness.

Crosslinked polymer fine particles composed of fine resin particles may be added to the above-mentioned glitter coating, and this addition effectively functions to control the viscosity and improve the appearance of the coating film.
The crosslinked polymer fine particles are obtained, for example, by suspension polymerization or emulsion polymerization of an ethylenically unsaturated monomer with a crosslinkable copolymerization monomer in an aqueous medium, and the obtained fine resin particle dispersion liquid is subjected to solvent substitution and azeotropic distillation. , A method of removing water by centrifugal separation and drying, a low SP organic solvent such as a fatty acid hydrocarbon or a high SP organic solvent such as an ester, a ketone and an alcohol, but a non-aqueous organic solvent in which a monomer is dissolved but a polymer is not dissolved A method of copolymerizing an ethylenically unsaturated monomer and a crosslinkable copolymer, and dispersing the obtained fine resin particles (copolymer) therein (referred to as NAD method or precipitation method) Manufactured by. In addition, JP-A-58-1
The method of using a water-soluble resin having a zwitterionic group described in JP-A-29066 may be adopted.

The crosslinked polymer fine particles to be added preferably have an average particle size in the range of 0.01 to 10 μm. For example, AZS797 and AZS597 manufactured by Nippon Paint Co., Ltd.
Are preferably used. Compounding amount is 100 resin solids
The amount is preferably 0.1 to 20 parts by weight, more preferably 0.5 to 10 parts by weight, based on parts by weight.

In addition, various additives that are commonly used in conventional coating compositions can be optionally added to the glitter coating. Examples of the additives include anti-settling agents such as oxidized polyethylene wax and polyamide wax, curing catalysts such as dodecylbenzene sulfonic acid, benzotriazole-based ultraviolet absorbers, benzophenol-based antioxidants, silicones and organic polymers. Surface modifiers, anti-sagging agents, thickeners and the like. These components can usually improve the performance of paints and coating films in an amount of 5 parts by weight or less based on 100 parts by weight of the coating film forming resin.

The top coat is generally of an organic solvent type, but is not limited to this, and various forms such as a non-aqueous dispersion type, an aqueous solution type and an aqueous dispersion type can be used. The solid content at the time of production is preferably 30 to 70% by weight. At the time of coating, it is diluted with a solvent such as an organic solvent or water to a coating-appropriate viscosity, and the solid content during coating is 10
-50% by weight is preferred.

Then, a clear coating is formed by applying a clear coating on the surface of the glitter coating by a wet-on-wet method. As the clear paint, a transparent resin which is generally used and is similar to the vehicle resin used for the glitter paint is used. The glitter coating film and the clear coating film are simultaneously cured by a 2-coat 1-baking method to form a composite coating film. The preferable dry film thickness of the clear coating film to be formed is 30 to 60 μm.
m.

[0025]

The method for forming a design coating film of the present invention is to form a glittering coating film by applying a coating material containing a glittering pigment component on the surface of a colored base coating film having a hue of a certain level or more, and then to form a clear coating film. It consists of a step of forming, but particularly when a bright pigment component having a composition composed of titanium dioxide-coated flaky sulfate barium sulfate and titanium dioxide-coated mica pigment mixed in a specific ratio range as a bright pigment is selected and used, the desired action and effect are obtained. There are major components that bring about. That is, the refractive index of barium sulfate as the base material of the bright pigment is 1.64, which is close to the refractive index of 1.54 to 1.59 of mica as the other base material. Therefore, the titanium dioxide-coated flaky barium sulfate pigment is
It shows a high degree of transparency inside the coating film, and since the substrate is a single crystal and contains no impurities, the pigment itself is not turbid. On the other hand, the titanium dioxide-coated mica pigment functions to complement the phenomenon of weakening of the luster caused by the titanium dioxide-coated flaky barium sulfate pigment.

The mutual complementing function of such pigment components is such that the titanium dioxide-coated flaky sulfate (P1) and the titanium dioxide-coated mica pigment (P2) have a weight ratio (P1 / P2) of 95/5 to 50 /.
When it is set in the range of 50, it is effectively exhibited, and coloring is achieved without causing a turbidity in the bottom, blurring of whiteness, and color unevenness in a state in which a decrease in the brightness and saturation of the colored base coating film is minimized. It produces a pearlescent luster that clearly reproduces the hue of the background. Therefore, it is possible to always form a glittering coating film having excellent design.

[0027]

EXAMPLES Examples of the present invention will be specifically described below in comparison with comparative examples. Titanium dioxide coated thin film barium sulfate pigment (P1) applied in each example, titanium dioxide coated mica pigment (P1)
2) and the coating film-forming resin were the following commercial products. In addition,
Indications of the above components in Tables 1 to 7 are based on the symbols shown before each component. The blending amount is shown in parts by weight of the solid content.

Flake-shaped barium sulfate pigment coated with titanium dioxide
(P1); (P1a) Nihon Koken Kogyo Co., Ltd., "Genestar 710"
1 "(silver color) (P1b) Nihon Koken Kogyo Co., Ltd.," Genestar 720 "
1 ”(Interference Gold) Titanium dioxide coated mica pigment (P2); (P2a)“ Iriodin 103WII ”manufactured by Merck Japan Ltd.
(Silver color) (P2b) "Iriodin 205WII" manufactured by Merck Japan Ltd.
(Interference Gold) Iron oxide-coated mica pigment (P3: for comparative example); (P3) Merck Japan KK-made "NP Lasset WII" film-forming resin; acrylic resin is styrene / methylmethacrylate /
Ethyl acrylate / hydroxyethyl methacrylate / methacrylic acid copolymer, number average molecular weight 2000
0, hydroxyl value 45, acid value 15, solid content 50%. Melamine resin is "Uban 20SE" manufactured by Mitsui Toatsu Chemicals, Inc.
(60% solids) was used.

Examples 1 to 16 and Comparative Examples 1 to 17 A 0.8 mm thick dull steel plate base material treated with zinc phosphate was coated with a cationic electrodeposition coating [Nippon Paint Co., Ltd., "Power Top U- 50 "] was applied so that the dry coating film had a thickness of 25 μm, and then baked at 160 ° C. for 30 minutes. Polyester / melamine resin-based intermediate coating on this electrodeposition coating surface [Nippon Paint Co., Ltd., "Olga S-90 Sealer"]
Blue, yellow, green, red, blue-green,
Was applied by air spraying so that the dry coating film would be 40 μm, and baked at 140 ° C. for 30 minutes to prepare a test plate. The hue of each of the formed colored base coating films was measured together with the Munsell color chart (displayed in the upper margin of Tables 1 to 7).

Flake-shaped barium sulfate pigment coated with titanium dioxide
(P1) and titanium dioxide-coated mica pigment (P2) in a predetermined weight ratio (P
1 / P2), the blending amount of the bright pigment with respect to 100 parts by weight of the resin solid content is blended in a ratio shown in the content of each table, and the blending resin is added to the coating-forming resin [acrylic resin / melamine resin = 80/20 (solid Min)]] to prepare a glitter coating. The viscosity of this glittering paint was adjusted with a solvent (toluene / xylene / ethyl acetate / butyl acetate = 70/15/10/5) at 20 ° C. for 12 to 15 seconds in a Ford cup # 4, and a dry coating film was obtained. Is 15
It was coated on the surface of the colored undercoating film of the test piece so that the thickness became μm.
Electrostatic coating machine [Ransburg Gemma, Auto
REA] at an atomization pressure of 2.8 kg / cm ² , and the booth atmosphere during coating was maintained at a temperature of 25 ° C. and a humidity of 75%.

After setting for 3 minutes after painting,
An acrylic / melamine resin-based clear coating [manufactured by Nippon Paint Co., Ltd., “Superlac 0-100”] was applied by wet-on-wet so that the dry film thickness was about 35 μm. Then, after setting at room temperature for about 10 minutes, baking was performed at 140 ° C. for 30 minutes.

With respect to the coating film formed, CIE
Optical measurement of hue in (L ^* , a ^* , b ^* ) display system,
Also, the feeling of cloudiness in the bottom, the feeling of white blurring, the feeling of glittering, and the unevenness of color by visual observation were evaluated. The results, for each intermediate coating hue of the colored base coating, the type of titanium dioxide-coated flaky sulfate barium sulfate pigment (P1) and titanium dioxide-coated mica pigment (P2) of the glitter coating, the compounding ratio (P1 / P2), the resin Tables 1 to 7 are compared with the content (parts by weight) of the bright pigment based on 100 parts by weight of the solid content.
It was shown to. In addition, Tables 6 and 7 are examples in which the lightness or saturation of the coating film for the colored underlayer deviates from the requirement range of the present invention.

Measurement of hue in CIE (L ^* , a ^* , b ^* ) display system; gonio-spectrophotometer [Murakami Color Research Laboratory, G
CMS-3 type, D65 light source, 2 degree field of view], the light source is illuminated at an angle of 45 ° from the perpendicular to the measurement sample on which a coating film is formed as shown in FIG. 1, and the light source is emitted from the specular reflection direction of the illumination angle. 15 ° to the side (light receiving point H in the highlight direction) and 11
The light was simultaneously received at an angle of 0 ° (light receiving point S in the shade direction). This was subjected to color measurement by a lightness-saturation L ^* a ^* b ^* color system to measure lightness (L ^* ) and saturation (C ^* ). C ^* was calculated by {(a ^* ) ² + (b ^* ) ² } ^1/2 . ΔL ^* in the table
And ΔC ^* are the differences from the colored base coat.

Criteria for visual evaluation / evaluation: The criteria for visual evaluation / evaluation are as follows. Bottom turbidity: ◎ None at all ○ Almost none □ Slightly △ Yes × strong White blur: ◎ None at all ○ Almost none □ Slightly △ Yes × Strong glitter: ◎ Very strong ○ Strong □ Slightly weak △ None Color unevenness: ◎ Very strong ○ Strong □ Slightly weak △ Weak × None

[0035]

[Table 1] Table Note: S part indicates a shade part, and H part indicates a highlight part (hereinafter the same).

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

[Table 5]

[0040]

[Table 6]

[0041]

[Table 7]

From the results of Tables 1 to 7, the design coatings according to the examples formed according to the present invention have relatively ΔL ^* and ΔC.
The width of ^* becomes smaller, there is less turbidity and white blurring, and the excellent brilliance with no color unevenness is maintained. On the other hand, the comparative examples that deviate from the requirements of the present invention are inferior in at least one performance such as cloudiness, white blur, glitter, and color unevenness, and the color feeling of the colored substrate such as the coating film of Examples is deteriorated. It was not possible to express a clear pearlescent luster.

The preferred embodiments of the present invention are as follows. (1) A method for forming a design coating film, in which a colored coating film is baked and cured, and then a glitter coating film and a clear coating film are formed by a wet-on-wet method. (2) The weight ratio of titanium dioxide-coated flaky sulfate (P1) and titanium dioxide-coated mica pigment (P2) in the glitter coating (P1 / P
2) is a design coating film forming method in which 80/20 to 50/50. (3) A method for forming a decorative coating film in which the amount of the bright pigment in the coating film for the bright layer is 4 to 8 parts by weight per 100 parts by weight of the resin solid content. (4) The lightness of Munsell display system of the colored underlayer coating is 6
As described above, the method for forming a decorative coating film having a saturation of 4 or more. (5) The particle size of the titanium dioxide-coated flaky sulfate barium sulfate pigment (P1) in the bright layer coating is 5-100 μm in major axis and 5-3 in minor axis.
A method for forming a decorative coating film having a thickness of 0 μm. (6) Dry film thickness of colored underlayer coating film / bright layer coating film / clear coating film is 25 to 50 μm / 10 to 30 μm / 30 to 60 μm
A method for forming a decorative coating film having m. (7) Crosslinking in which the resin of the bright layer coating film is composed of at least one thermosetting resin selected from acrylic resin, polyester resin, alkyd resin, silicon resin and fluororesin, and amino resin and / or (block) polyisocyanate A method for forming a decorative coating film which is a combination of agents. (8) A method for forming a decorative coating film, which comprises forming a coating film of a colored undercoat layer on the electrodeposition coating layer which has been subjected to electrodeposition coating after chemical conversion treatment.

[0044]

As described above, according to the method for forming a design coating film according to the present invention, the flaky barium sulfate pigment coated with titanium oxide and titanium dioxide having a specific mixing ratio are provided on the coating surface of the colored underlayer having a certain hue. A multi-layer coating with a pearlescent luster that reproduces the color of a colored substrate without turbidity or color unevenness by sequentially performing the steps of forming a glitter coating containing a coated mica pigment and a clear coating. Can be obtained.
Therefore, it is extremely useful for the purpose of forming a high-quality multi-layer coating film on various types of substrates to be coated, including the exterior coating of automobile bodies, which requires excellent designability.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a hue measuring method in Examples.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Higashi Yanagida 6-1, Ichibancho, Tachikawa, Tokyo 2 Nikko Kogyo Co., Ltd. (72) Inventor Kazuhiro Kimura 6-1, Ichibancho, Tachikawa, Tokyo Address 2 Nippon Koken Kogyo Co., Ltd.

Claims (1)

[Claims] 1. A method for forming a design coating film, which comprises sequentially performing the following coating film forming steps (1) to (3). (1) Brightness is 3 or more and saturation is 2 in the Munsell display system
The above-mentioned colored base layer coating film forming step (2) The titanium dioxide-coated flaky sulfate sulfate pigment (P1) and the titanium dioxide-coated mica pigment (P2) having a weight ratio (P1 / P2) in the range of 95/5 to 50/50. (3) A bright layer coating film forming step containing 3 to 10 parts by weight of a bright pigment having a composition of (3) per 100 parts by weight of a resin solid content, and (3) a clear coating film forming step.